Fatigue testing machine



Aug 3, 1954 L.. A. sTRz-:BLow 2,685,195

FATIGUE TESTING MACHINE Filed Feb. 26, 1952 3 Sheets-Sheet l INVENTOR. im! 5617,06/ Jy/291366 ow www Q /A-:

Aug- 3, 1954 L. A. sTREBLow 2,585,195

FATIGUE TESTING MACHINE l Filed Feb. 2e, 1952 s sheets-sheet 2 ZNVENTOR.

Aug. 3, A1954 Filed Feb. 26. 1952 L. A. STREBLOW FATIGUE TESTING MACHINE 3 Sheets-Sheet Y WOR.

Patented Aug. 3, 1954 FATIGUE TESTING MACHINE Lawrence A. Streblow, Kenosha, Wis., assigner to Fansteel Metallurgical Corporation, a corporation of New York Application February 26, 1952, Serial No. 273,456

(Cl. 'i3-15.6)

7 Claims.

This invention relates to a fatigue testing machine and particularly to a machine for test ing specimens for tensile strength under predetermined fatigue conditions.

It is well known that materials which exhibit characterisitcs responses in tensile stresses have different behaviors depending upon the manner iniwhich the stress is applied. Thus, many materials have well-defined tensile stress characteristics when the stress is applied slowly and maintained upon the specimen continuously. However, these same materials may exhibit entirely different response characteristics if the tensile stress is applied suddenly or if the magnitude or" the stress is varied cyclically over a period of time. Most materials develop a so-called fatigue characteristic which results in breakdown at stresses.

Many materials and parts using various pieces of mechanism are subjected to cyclic stresses or stresses which vary from zero to a maximum in a short period of time. In the design oi such pieces, a knowledge of the fatigue characteristics of the material is of great importance. Not only must a material, of which a part is made, be tested under fatigue conditions but it may even be necessary to test the designed part under fatigue conditions as a check upon the design. Such fatigue conditions not only include the cyclic application of stresses, but also include the application of such stresses to the specimen under controlled temperature conditions.

Machines for testing specimens under fatigue conditions are complicated and, in many instances, the results of the test are questionable. This invention makes it possible to provide a simple mechanism for tensile testing accurately specimens under controlled fatigue conditions.

A machine embodying the present invention relies upon the fact that spring steel is almost perfectly elastic, providing the elastic limit of the steel is not exceeded. Within the limits of elasticity of spring steel, the spring characteristics remain substantially independent of the `manner inwhich stresses are applied to the spring and, within limits, of the cyclic variation of the stress. Thus, the present invention is based upon the fact that l-Iookes Law applies accurately to a spring steel member.

In general, a machine embodying the present invention has a calibrated spring steel member arranged to transmit any desired stress upon a sample, and has means for cyclically varying the value of the stress between desired limits at a desired' rate. It is understood that the stress limits and rate of application or variation of stressare such that no part of the spring steel member has stresses created therein which exceed the elastic limit of the steel. As the elastic properties of spring steel are widely known and have been thoroughly investigated, anyone skilled in the art may readily determine when the operating limits of the spring steel are approached. For accurate results, it is best to operate the spring steel well within its elastic limits and avoid any close approach to the elastic limit,

The new machine embodying the present invention has means for controlling the stress limits transmitted by the calibrated spring steel member. Additionally, a machine embodying the present invention has means for controlling the temperature of the specimen being tested, and also permits of measurements or" such constants as conductivity of electricity in the specimen during testing.

In general, the new testing machine has means for mounting a specimen .so that a calibrated spring steel member exerts tension upon the specimen. Cyclic means, whose speed may be controlled, are provided for varying the value of the tensile stress applied to the specimen, the variation ranging from a maximum value down to a zero value. Means are also provided for heating the specimen to any desired temperature during test.

In order that the invention may be understood; an exemplary embodiment of the invention will new be explained in connection with the drawings, it being understood, however, that substantial variations in construction and detail may be made'without departing from the spirit of the invention.

Referring, therefore, to the drawings:

Figure 1 is a front elevation of a machine embodying the present invention;

Figure 2 is a View along line 2--2 on Figure l;

Figure 3 isa detail of a portion of the machine shown in Figure 1, when the parts are in position for zero tension on the specimen under test;

Figure 4 is a View similar to Figure 3 but showing the machine in a diierent position, where maximum tension upon the specimen is provided;

Figure 5 is a detail on line 5-5 of Figure 3 and illustrates the means for indicating the tension on a specimen;

Figure 6 is a perspective detail of the clamping block for retaining an end of a specimen.

The machine comprises base is uponk which the variousparts are suitably attached or bolted for support. Base luis preferably of steel a1- though it may be oi concrete, wood, or other material, and has suflicient strength for supporting the various parts of the machine under operating conditions.

Extending upwardly from base 1B are columns i i and i2. These columns are preferably of steel and may have any desired shape and dimension. Columns I I and I2 may be attached to base I0 in any suitable fashion, as by bolts or rivets as shown. The tops of the columns are maintained rigidly in predetermined relation by transverse member ifi. Centrally of member Hi is supporting block I to which one end of the specimen under test is normally attached. Supporting block i6 is preferably o1" metal and may consist of a massive block of copper, aluminum, or any other metal. Copper is preferred because of its excellent heat and electrical conductivity. As will be shown later, it may be desirable to transmit heating current through the specimen under test.

When electric current is passing through the specimen, the potential of block IB is above ground and accordingly, it is necessary to insulate this block from the frame as a whole. Aocordingly, member Iii, extending across columns I l and i2, may consist of insulating material such as transite, or may consist of steel insulated from block It or from columns II and I2, or from both. Thus, for example, since many insulators have excellent compression characteristics, suitable sheets of insulating material, such as bakelite, glass or the like, may be interposed between the tops of columns II and I2 and the opposed parts of member I4. Insulating bushings for the bolts for maintaining the parts in position may also be provided. Inasmuch as the electrical insulation of one part of a machine from another part of a machine is well known and is widely used, various means for insulatingly mounting block I6 with respect to columns II and I2 will occur to those skilled in the art.

Block l-S itself is made in two parts and generally resembles a vise. Thus, referring to Figure 6, for example, it will be seen that block IS generally consists of massive base member Il and movable block Iii cooperating with part I3 of the block. Bolts 28, passing through apertures in movable block I3 and engaging threaded apertures in part I9, may be used for drawing the parts together. idly clamped between the two parts of the block.

Disposed vertically below block I 6 is a companion block 22, which is generally similar1 to block it in construction. Block 22 is rigidly clamped between a pair of flat plates 2li and 25 I (Figure 2), these plates being either of metal or of insulating material, in case it is desired to insulate block 22 from the machine. If these plates are of insulating material, it will be necessary to design the plates with regard to the nature of material and dimensions so that there will be no question of the ability of these plates to transmit the stresses to be impressed upon the specimen under test.

Plates 24 and 25 are maintained in predetermined position by bolts 26 and 2l. Bolt 26 passes through plates 2i; and 25 and also passes through the lower portion of block 22. Bolt 21 passes through insulating plates 24 and 25 and yoke 28 of guide frame 3c, preferably of steel. Between the plates, forming yoke 28 of guide frame 30, and below bolt 2'! is bar 3I of steel or other material, extending outwardly toward columns II and I2. Bar 3l is loose and has its ends apertured to accommodate bolts 32 and 33 respectively. Bolt The specimen may thus be rig- 32 is carried by bracket 34 on column II, while bolt 33 is similarly carried by bracket 35 on column I2. Brackets 3d and 35 are arranged so that they may be adjusted vertically with respect to the columns and tightened in position. Bolts 32 and 33 have wing nuts 3T threaded over them below bar 3 I. As is evident from Figure l, in case of a failure of the specimen, bar 3l will cooperate with the wing nuts to limit the drop of block 22 and all the mechanism movable therewith.

Yoke 29 forms the top end of guide frame 30 consisting of vertical members 32 and 33 and bottom member 34a. Bottom member 34a is apertured to accommodate elongated threaded rod 36 passing longitudinally of the frame. Washer 39 is shaped as shown to provide a central boss for engaging the end coil or coils ci spring lll. The coil spring is calibrated so that the strain and stress relationships are known accurately. Usually, such springs are calibrated by determining the amount of force required to compress the spring to accomplish a unit percentage reduction in unstressed length thereof.

The top end of coil spring il rests against nut member i3 having a central boss similar to washer 39. Nut member .153 engages rod 3B at a threaded part thereof. Above nut member i3 is lock nut lili and between the lock nut and the nut member is clamp-ed bracket 15. Bracket i5 is connected to a spring scale and also to a counting mechanism, both or these to be described later.

vertically aligned with guide frame 323 and disposed below it is a second guide frame 5i) consisting of top portion 5I, sides 52 and 53, and bottom 54. Bottom guide frame 59 is generally similar to top guide frame 3i?. Thus, top portion 5i of the bottom guide frame is apertured to accommodate rod 35. Resting against the bottom surface of portion 5I is dished washer 51, maintained in position by nut 58 engaging the threaded end of rod 3G.

Coil spring 53 need not be calibrated and serves to transmit the tensile stress to which the specimen is to be subjected. It will be evident that by adjusting the positions of the nuts at the two ends of rod 36, any desired degree of compression in the two springs may be provided. The compression of the two springs when the frames are in the position shown in Figures 1, 2 and 3, is taken up completely by rod so that no tension outside of the two guide frames is provided.

Suitable means are provided for vertically re ciprccating lower guide frame 5i! from the position shown in Figures l, 2 and 3. It is clear that ii bottom guide frame 5@ is pulled vertically downwardly, as seen in Figures 1 and 2, there will be a tendency to compress coil spring 5S. The compression of coil spring 58 will communicate a tensile force upon rod 36 which will transmit this force through calibrated coil spring @I to part 315i of the top guide frame. Yoke end 29 oi' top guide frame 30 will transmit this downward force to lower clamping block 22 and cause tension to be applied to the specimen on test.

The means for reciprocating the lower guide frame may consist of any suitable mechanism, the amplitude and frequency of the reciprocations being adjustable to suit desired testing conditions.

Disposed below bottom guide frame 5B is bar 58 resting against the bottom part 54 of the guide frame and urged upwardly by coil springs and EI. Coil spring 5I] and 5I are maintained in Vertical position by guide bolts 62 and 63 carried by the base of the machine and passing through suitable apertures in bar 58.' Nuts fit and $5 above bar 58 determine the upward limit ci travel of bar 58.

Engaging the top surface of part be ci the lower guide frame is roller 'it carried by crank arm il. Crank arm 'ii is preferably adjustabt`l so that the amplitude' of vertical movement oi the-guide frame may be adjusted, wittoany desired value. Crank arm 'il is driven by shaft i2, and this shaft in turn is driven any suitable source of power, such as an electric motor and suitable speed-reducing means. Shaft i2 may be driven at speeds of the order of about one revolution per minute to teen revol per minute. However, the speed range Amay be above or below the heures given, depending upon the lsize of the testing nature or" specimens, and other factors.

Rotation or shaft lil will cause cranl ari to Aturn and this will result in roller preesin down upon lower part of the trarre and Torcin the lower guide frame downwardly from th position shown in Figure l. shown in Figur 4, downward movement of the crank will caueJ the lower guide frame to be pulled downwardly away from the upper guide trarne. 5S will be kept tightly against the bottom or the lower frame. By virtue of the system or springs, it is clear that irrespective of whether the lower guide frame moves downwa or upwardly, contact will be maintained between bottom part of the lower guide fra-ine and roller lil. t is stood, oi course, that the rotation. of the crank arm is slow enough so that the spring system will be able to follow the movements of the guide frame.

Means are provided for indicating the amount of tensile stress to which the specimen is subn jected. Thus, referring to bracket 46, the free end thereof carries rack Sil. 3i? extends vertically down and cooperates with pinion ai coupled to pointer Si? playing over scale on indicating plate all. The iree end of bracket in is also connected by elastic cord '36 to operatin arm 8l or" counter S3. Thus, every time thT bracket 4S is pulled downwardly, counter 5B actuated. It is also clear that when bracket it moves downwardly, the rack pinion arrangement will cause pointer 82 to run over scale In order to maintai 'seth of travel ci bottom guide frame 55.2 in a ed ver rollers: 86o. carried by mounting plates ported on the frame, are provided. Rollers @da cooperate with rails 3S rigidly supported on columns il i2.

Means are provided to the testing operation in case of failure oi the spec en Thus, arm extends iron1 one of the columns, such as column it, for eale, toward top g ide frame 39 and normally is di posed just below the yoke portion of this guide frame. Eil coupled to switch Si, which controls the circ for the motor for operating the testing machine. In case of failure oi the specimen, lower n m ce o:

ing block 2E and the yoke attached thereto will drop. The amount oi will be determined by the clearance between bar Si and the bottom oi the clamping block 22, and in any event,

clearance will be more than ample to perm: the bottom of the yoke to drop arm il@ and ti ip the switch.

In case it is desired to heat the specimen assuming that the specimen is conducting, heavy conductors 93 and afl are connected respectively to clamping blocks it and 22, these conductors being connected to a suitable source of current.' Thus, the specimen under test may be heated to. any desired temperature while it is being tested.

To prevent overheating of the clamping blocks for retaining the ends of the specimens, cooling pipes and 9? are provided for the two clamping blocks. These pipes are connected by rubber hose et, and the two pipes and hose may be supplied with suitable cooling uid, such as oil or water.

Specimen E98, shown in Figure l, consists of an elongated member. However, other specimens having diierent shapes may be used and it understood that clamping blocks and 22 may be` supplied with suitable attachments to accomino-date different specimens. lt is possible, by suitable adjustment of coil springs lil, 55, and ci, to have a compression iorce exerted upon a specimen when the mechanism is the position shown in Figure 3. When the guide frames are separated as shown in Figure fi, the compression force upon thespeciinen may be reduced to zero or may even change to tension. Thus, it may be possible to test disc valves, diaphragme, expansible bellows, and various types of articles which are subject to fatigue conditions.

i claim:

l. A machine for testing a specimen under conditions inducing fatigue in specimen, said machine comprising a base, means on base for rigidly supporting one part of a specimen under test, a spring having calibrated elastic characteristics, means for supporting another part of a specimen, means coupling one end of said spring to said last named specimen-supporting means, a second spring, a 1igid rod extending through both springs and secured at its opposite ends to one end of each of said springs, means for impressing a force upon the second spring to be transmitted successively to said rod, said Erst spring, and through said coupling means to said specimen, and indicating moans coupled to said iirst spring for showing the amount of force applied to said first spring to be transmit-ted to said specimen.

2. In a machine for testing a specimen under conditions inducing fatigue, said machine coinprising a base, spaced means carried by said base for supporting a specimen under test, a pair of relatively movable frames, coupling means including a calibrated spring for transmitting 'force between said two frames, means for attaching one o said frames to one specimen-supporting means, a spring mounted in the other of said frames, means for impressing a Variable force on said second mentioned frame, means connected to said two springs for transmitting said force from said second mentioned trame through said second mentioned spring to said calibrated spring, calibrated spring serving to transmit all the force to said rst mentioned frame, and means coupled to said calibrated spring for indicating the amount of force transmitted by said spring.

3. A machine for testing a specimen under conditions inducing fatigue, said machine comprising a base, means on said base for supporting one part of a specimen, a pair of guide frames in axial alignment, a longitudinal rod passing through said frames, spring means coupling said rod to said frames, said frames, springs and rod together forming a force-transmitting path from one frame through said spring means and rod to the other frame whereby longitudinal force on one frame is transmitted to the other frame, said two frames being relatively movable to each other axially, means on one frame for supporting another part of said specimen, means for impressing a cyclically varying force upon the other frame to move the same axially through a range of movement, said one frame thereby having cyclic forces transmitted to it to be impressed upon said specimen, and means coupled to one of said spring means for indicating the spring response to the force transmitted by it, said spring being calibrated so that the force impressed upon said specimen is known.

4. The machine according to claim 3, wherein said one spring is a calibrated coil spring and wherein said means for indicating the response of said spring comprises means for indicating the change of length of said spring.

5. The machine according to claim 3, wherein means responsive to the movement of said one frame are provided for stopping the operation of said testing means whereby, upon failure of a specimen and consequent movement of said one frame, said means Will operate to stop the machine.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 902,497 Landgraf et al. Oct. 27, 1908 1,156,726 Upton et al. Oct. 12, 1915 1,409,842 Foster Mar. 14, 1922 1,695,046 Hippensteel Dec. 11, 1928 1,888,755 Barr et al. lNoV. 22, 1932 1.907,907 Varney May 9, 1933 

